Question

How can iodine \((Z=53)\) have a higher atomic number yet a lower atomic mass than tellurium \((Z=52) ?\)

Short answer

Isotopes of tellurium have more neutrons compared to isotopes of iodine, making tellurium's atomic mass higher.

Step-by-step solution

Identify Atomic Numbers and Mass Numbers

Tellurium has an atomic number of 52 and iodine has an atomic number of 53. This means tellurium has 52 protons and iodine has 53 protons.

Understand the Concept of Atomic Mass

Atomic mass is primarily determined by the number of protons and neutrons in the nucleus. Electrons contribute very little to the atomic mass.

Explain Isotopes

Isotopes are atoms of the same element with different numbers of neutrons. This means that the same element can have atoms with different atomic masses.

Compare Common Isotopes

The most common isotope of tellurium is Tellurium-128, which has 52 protons and 76 neutrons, giving it an atomic mass of 128. The most common isotope of iodine is Iodine-127, which has 53 protons and 74 neutrons, giving it an atomic mass of 127.

Analyze the Mass Numbers

Even though iodine has a higher atomic number than tellurium, the mass number (sum of protons and neutrons) of tellurium-128 is higher than that of iodine-127. Therefore, tellurium can have a higher mass even if it has fewer protons.

Conclusion

The atomic mass can be higher in tellurium due to the number of neutrons. This explains why iodine can have a higher atomic number but a lower atomic mass than tellurium.

Key concepts

Atomic Number

The atomic number of an element is defined as the number of protons in the nucleus of an atom. This number is crucial because it determines the identity of an element. For instance, iodine has an atomic number of 53, meaning it has 53 protons. Similarly, tellurium has an atomic number of 52, meaning it possesses 52 protons. Each element on the periodic table has a unique atomic number. The atomic number also dictates the element's position on the periodic table, with elements ordered sequentially by increasing atomic number.

Atomic Mass

Atomic mass, also known as atomic weight, is the total mass of protons, neutrons, and electrons in an atom. However, the mass contributions from electrons are negligible. Therefore, the atomic mass is largely determined by the number of protons and neutrons in the nucleus. Importantly, even though iodine has more protons (53) than tellurium (52), tellurium can have more neutrons, leading to a higher atomic mass. For example, tellurium’s common isotope has a mass number of 128, whereas iodine’s common isotope has a mass number of 127. This difference illustrates that atomic mass is not solely dependent on the number of protons.

Isotopes

Isotopes are versions of the same element that have the same number of protons but different numbers of neutrons. This difference in neutron number affects the atomic mass. For example, the most common isotope of tellurium is Tellurium-128, with 52 protons and 76 neutrons. Iodine’s common isotope, Iodine-127, has 53 protons and 74 neutrons. Despite having a greater atomic number, iodine can have a lower atomic mass than tellurium due to these neutron differences. Isotopes are critical in various fields, such as medical diagnostics and archaeological dating.

Neutrons

Neutrons are subatomic particles found in the nucleus of an atom. They carry no electric charge, but they play a vital role in determining the atom's mass. The number of neutrons can vary even among atoms of the same element, leading to the formation of different isotopes. For instance, while iodine and tellurium have nearly the same number of protons, the number of neutrons differs. Tellurium-128 has 76 neutrons, whereas Iodine-127 has 74. This variation in the number of neutrons affects the atomic mass of the respective isotopes. Neutrons also help stabilize the nucleus by offsetting the repulsive forces between protons.

Protons

Protons are positively charged subatomic particles located in the nucleus of an atom. The number of protons, known as the atomic number, determines the identity of the element. For example, iodine has 53 protons, and tellurium has 52. Protons not only define the element but also influence the chemical properties and behavior of the atom. The presence of each proton adds to the atomic mass. However, as previously noted, the atomic mass is also heavily influenced by the number of neutrons. Thus, even with fewer protons, an element like tellurium can have a higher atomic mass due to a greater number of neutrons.